The present invention relates to electrical apparatus and, more particularly, to electrical capacitors having protective means for removing electrical capacitors from their electrical circuits in the event of a failure thereof.
The present invention is particularly, but not exclusively, applicable to capacitors employed for power factor correction in such apparatus as appliance motors and lighting systems. In the past, such capacitors have been enclosed in liquid-filled sealed cans which are conventionally enclosed within housings of, for example, ballasts for high-intensity discharge lighting systems. The capacitors are frequently included within the asphalt, or other potting compounds, within the ballast housings.
The active portion of such a capacitor typically consists of a roll of a metallized dielectric material such as, for example, a metallized polypropylene or polyester film sealed within the can. A liquid dielectric fills a substantial portion of the space between the capacitor roll and the can. External terminals pass sealingly through the can to make electrical contact with the metallized surface.
When such a capacitor near the end of its life, its failure modes tend to generate heat. The generated heat, in turn, tends to accelerate the failure until a thermal runaway condition produces an extremely rapid temperature rise accompanied by the generation of a quantity of gas which is capable of exerting a substantial pressure on the interior of the can. The gas pressure is frequently sufficient to rupture the can and to thereby release the liquid dielectric. When the capacitor is included within the asphalt potting compound the released liquid may mix with the potting compound and the liquid mixture of dielectric and potting compound may escape from the ballast housing and stain or damage external portions of the fixture containing the capacitor as well as nearby furniture and other objects. In addition to the above staining problem, the rupture may occur explosively with the accompanying production of an explosive bang and the emission of substantial quantities of smoke. Such occurrences have been known to alarm the uniformed observer.
One prior solution for preventing the degradation of a sealed capacitor to a danger point is disclosed in the Rayno U.S. Pat. No. 3,377,510, of common assignee with the present invention, the disclosure of which is herein incorporated by reference. The Rayno invention employs the bulging of the cover of the capacitor can in response to internal pressure to break an electrical connection with the capacitor roll within the can. Breaking the electrical connection effectively removes the capacitor from the electrical source and halts the further generation of failure-derived heat.
A further prior solution is disclosed in the Flanagan U.S. Pat. No. 4,106,068, of common assignee with the present invention, the disclosure of which is also herein incorporated by reference. The Flanagan invention adds a plurality of fulcrum members operating on a bridge member within the liquid filled can to improve the leverage applied to the internal connections in order to break more than one of the internal connections, and thereby more positively to remove the capacitor roll from the source of electrical power.
Suitable capacitors for the above-recited and other applications may now be fabricated in a manner which eliminates the need for the internal liquid dielectric. Such capacitors, hereinafter referred to as dry capacitors, no longer require the sealed can to prevent leakage of a liquid dielectric. Dry capacitors typically consist of a roll of one or more metallized dielectric sheets enclosed within a housing of a plastic resin.
When a dry capacitor fails, it generates a substantial quantity of gas which may rupture the resin housing. The rupture may occur slowly or explosively and is typically accompanied by an emission of smoke. In addition, the temperatures generated in a failure may be sufficient to exceed the flash point of the capacitor or its environment. The place on the housing where the rupture may occur is not readily predictable. Since the need for a sealed can has been eliminated by the omission of a liquid dielectric, the bulging of the sealed can, which the Rayno and Flanagan inventions relied on for circuit interruption, cannot be relied on for early detection of the failure condition and interruption of the electrical circuit.
One attempt to provide an interruptor for such a dry capacitor is disclosed in U.S. Pat. No. 4,283,750 in which a crease is provided in a metallic case within which the plastic resin is cast. The crease is intended to function as a type of bellows which expands under gas pressure to sever an internal connection. The unpredictability of the location at which the gas pressure may develop limits the applicability of the disclosed device. In tests of capacitors having resin casings cast within metal cans it was discovered that the internal gas pressure has no difficulty rupturing both the resin casings and the metal cans in the immediate vicinity of the failure without migrating substantially within the can in any predictable direction. This would seem to indicate that operation of the disclosed device may depend on fortuitous location of the failure site in the vicinity of the crease.
A further possibility is disclosed in a paper entitled "Some Recent Developments in Low and High Voltage Power Capacitors" which was presented at the FOURTH BEAMA INTERNATIONAL ELECTRICAL INSULATION CONFERENCE held in Brighton, England in 1982 by Curtis, Edwards and Constandinou. Using a common European technique normally applied to liquid-filled capacitors in sealed cans, the paper discloses a corrugated bellows portion of a metallic can within which a cured thermosetting resin casing is cast about a capacitor roll. The bellows portion provides a region on the can which is longitudinally expandable under the influence of internal gas pressure. The longitudinal expansion of the bellows portion is employed to sever one or more notched wires therein and to thereby interrupt electrical connection to the capacitor roll. In one illustration, the paper discloses a metallized film capacitor having a cured thermoset resin inside the can. In order to permit the gas to reach the bellows portion of the can, the paper mentions, without specifications thereon, the use of an outer wrap on the capacitor roll which includes a gas conducting liner.